We study a group of related inborn errors of metabolism, the hereditary methylmalonic acidemias (MMA), disorders of intracellular cobalamin metabolism and propionic acidemia (PA), in the clinic and laboratory. Affected patients are medically fragile and suffer from multisystemic complications such as severe metabolic instability, stroke of the basal ganglia, pancreatitis, chronic kidney disease, growth impairment, osteoporosis, cardiomyopathy and developmental delay. Despite intensive study since the early 1960s, few patients with MMA or PA have survived into adulthood. The evvidence for effectiveness of current medical therapies is lacking, a fact that stands in stark contrast to the practice of screening all US newborns for these disorders. Because patients with MMA and PA display pathology that can be seen in many common conditions, such as vitamin B12 deficiency, stroke syndromes, pancreatic dysfunction, diabetes, chronic kidney disease, osteoporosis, cardiac dysfunction and obesity, it is likely that the careful elucidation of patient phenotypes will provide new insights into the pathophysiology and treatment of these more prevalent disorders. The translational research program has three major interrelated goals. Goal 1 is to define the natural history of the conditions using clinical research; Goal 2 is to investigate the disorders in the laboratory using metabolic, genetic and genomic approaches, including animal modeling; and Goal 3 is to develop new treatments for the disorders, focusing on gene therapy strategies which include mRNA therapy, AAV gene delivery and genome editing. The clinical characterization of patients with methylmalonic acidemia (MMA) and related disorders is accomplished via a dedicated NHGRI natural history study, Clinical and Basic Investigations of Methylmalonic Acidemia and Related Disorders (ClinicalTrials.gov Identifier: NCT00078078). Through this clinical protocol, we have continued to enroll patients with MMA and cobalamin metabolic disorders and have evaluated > 200 affected individuals. The NIH MMA protocol has accrued the largest single center cohort of MMA patients in the world. A more recent endeavor to study propionic acidemia was initiated in late 2016 though NHGRI protocol Natural History, Physiology, Microbiome and Biochemistry Studies of Propionic Acidemia (ClinicalTrials.gov Identifier:NCT02890342). Like the methylmalonic acidemia (MMA) protocol, this clinical effort is unique in that it is the only dedicated natural history study being conducted on PA that utilizes an intensive dedicated hospital visit to perform detailed clinical phenotyping. At present, 30 patients have been evaluated and another 10-15 are expected to enroll before the end of 2019. This cohort, like that of MMA, is the largest diverse PA patient cohort assembled to date, and continues to expand in size. We have continued to focus on the clinical characterization of patients to inform our understanding of these common organic acidemias. We have written a number of contemporary reviews on MMA, PA and cobalamin disorders, including a recently updated Genereview article on cobalamin disorders (Sloan et al, in press), and remain as opinion leaders in the debate regarding medical food use in MMA and PA (reference 1). Active efforts include defining the metabolic phenotype of MMA and PA patients with stable isotopes; the construction of an integrated clinical outcomes database for MMA and PA; the analysis of cardiac phenotypes in cblC (in collaboration with NHLBI colleagues); the delineation of the natural history of PA; and the identification and validation of protein biomarkers that correlate with disease severity in MMA and PA. Translational investigations have continued to focus on preclinical experiments that will enable new treatments for MMA to reach the clinic. This past year, as part of a CRADA with Moderna Therapeutics, we developed the first systemic mRNA therapy for MUT MMA. Our paper was published in Cell Reports (reference 2) and designated by the Board of Directors and the Scientific Advisory Council of the Oligonucleotide Therapeutics Society as the December 2017 Paper of the Month. This report relied upon our proprietary mouse models of MUT MMA and isotopic monitoring methods to show, for the first time, that mRNA therapy could be a viable treatment for MMA. We generated a pseudoU-modified codon-optimized mRNA encoding human methylmalonyl-CoA mutase (hMUT), the enzyme most frequently mutated in isolated MMA, and encapsulated it into biodegradable lipid nanoparticles (LNPs). Intravenous (i.v.) administration of hMUT mRNA in two different mouse models of MMA resulted in a 75%85% reduction in plasma methylmalonic acid and was associated with increased hMUT protein expression and activity in liver. Repeat dosing of hMUT mRNA reduced circulating metabolites and dramatically improved survival and weight gain. Additionally, repeat i.v. dosing did not increase markers of liver toxicity or inflammation in heterozygote MMA mice. This new mRNA therapy has now been advanced by Moderna Therapeutics as a treatment for MMA, and a clinical trial is planned by the company. We continue to characterize knock-out and transgenic mouse models of disorders that present in our patient population, including MMAA deficiency, which causes a relatively common and severe form of vitamin B12 responsive MMA, combined malonic-methylmalonic acidemia due to acyl-coA synthase family member 3 (ACSF3) deficiency, cobalamin C deficiency (MMACHC), and propionic acidemia (PCCA, PCCB). A comprehensive paper reporting on new biomarker identification in MUT MMA, validation studies in patients and use in gene therapy is under review (Manoli et al, under preview). In the next year, we will continue to characterize the mutant mice using genomic, proteomic and metabolomic analyses, then test new therapeutics, such as mRNA therapy, AAV gene therapy, genome editing and microbiome manipulations. We have also initiated a parallel effort to model lethal metabolic disorders such as MUT MMA, cobalamin C deficiency, and propionic acidemia in zebrafish with the anticipation that the zebrafish models will be amendable to the facile testing of small molecules, possibly in a high throughput fashion. We have continued to focus on gene therapy as treatment for methylmalonyl-CoA mutase (MUT) deficiency, the most common and severe form of isolated MMA. We have developed new AAV vectors for MUT MMA, garnered intellectual property, licensed our vectors for commercial development, and partnered with two companies that are dedicated to bring gene therapy to the clinic. We have also created AAV gene therapies for cobalamin C deficiency and Niemann-Pick Type C (NPC) disease (with Dr William Pavan of the NHGRI), setting the stage for the development of gene therapy for several disorders within the NIH IRP. Along these lines, several patents related to MMA biomarkers have been filed, and CRADAs executed to support further development of new gene therapies.